Hall elements are magnetic field sensors which are based on the Hall effect and provide an electrical output signal which is proportional to a predetermined component of the magnetic field. A basic Hall device consist of a conducting material provided with at least four electrical contacts. In order to make use of the Hall effect, a current has to flow through the device. A bias current I is supplied via two of the contacts, which will be referred to as the current contacts. Two other contacts, referred to as the sense contacts, are typically placed on an equipotential line, to make the voltage difference between the sense contacts zero in the absence of a magnetic field. The principle of measuring a magnetic field component Bz using a horizontal Hall element is illustrated in FIG. 1. For a Hall readout, the current contacts A, C and sense contacts B, D alternate with each other. If a current I is applied to the current contacts A, C, and if an out-of-plane magnetic field Bz is applied to the device, a Hall voltage VH proportional to the applied magnetic field Bz will appear between the sense contacts B, D; in other words, VH=VB−VD.
A Hall sensor comprises a Hall element or a cluster of Hall elements and an electronic circuit for operating the Hall element(s) and for evaluating the output signals of the Hall elements. The Hall sensor is manufactured as an integrated circuit which is embedded in a semiconductor chip. The semiconductor chip is packaged in a housing. Hall elements have an offset which arises from process- and geometry-related variations. The offset can be effectively minimized by connecting a plurality of Hall elements in parallel (cluster) and/or by operating using the known spinning current method. This is known from numerous patent documents, for example, WO 0118556, EP 548391, and DE 4302342.
Hall elements can easily be integrated in semi-conducting devices, e.g. in CMOS technology, which implies that they can be combined with advanced on-chip readout circuitry. An implication of the Hall device being co-integrated with other (e.g. readout) devices, is that the Hall element needs to be electrically isolated from the substrate and other components. In integrated technologies, this can be accomplished by using reverse-biased PN-junctions.
In FIG. 2, a cross-section of an integrated horizontal Hall plate is shown, cut along the line where the excitation is applied (line through the contacts A and C in FIG. 1. By way of example, a CMOS process using a p-type substrate has been illustrated. The actual Hall plate then consists of the n-type material of an n-well. In this example, also a p-type covering layer (top shield) is illustrated on top, which is often provided for one or more of various reasons (improved shielding, less noise of the device, etc). In FIG. 2, both substrate and top shield are connected to ground (0 V). During Hall readout, a current I has to flow through the plate. For this purpose, node A and node C must be at a different voltage. By way of example, it is assumed here that the applied biasing method results in 3.0 V at node A, and 1.0 V at node C. As is well known from the theory of PN-junctions, at any transition between p-type and n-type material a depletion region is formed. The biasing is done in such a way that the PN-junctions are always reverse-biased. The reverse-biased transitions provide electrical isolation of the plate. The isolating depletion regions extend into the Hall plate, near the p-type substrate and the p-type cover (grayed areas in FIG. 2), and have a low number of free charge carriers (i.e. these regions can be considered as nearly perfect isolators). As a result, the effective thickness of the Hall plate is reduced. The actual size of the depletion zone varies in a non-linear way with the local (reverse) voltage over the PN-junctions. This reverse-voltage varies over the plate, being the largest at the node where the current enters (node A at higher potential), and the smallest at the node where the current leaves (node C at lower potential). As a result, the plate thickness (d1 in FIG. 2) at the high-voltage side is smaller than the plate thickness (d2 in FIG. 2) at the low-voltage side, implying a non-uniformity of the plate thickness in the direction from A to C. In other words, the thickness of the Hall plate is not constant, but varies over the plate. Unfortunately, when using current biasing, the effectively applied voltages (between nodes A and C) depend strongly on temperature, but also piezoresistive stress-effects and even the Hall effect itself affects the voltages (i.e. the voltages also vary in the X-direction of FIG. 2). Because these effects modulate the thickness of the plate, they affect the sensitivity and the linearity of the magnetic sensor.
An important characteristic of a Hall sensor is the (magnetic) sensitivity. Ideally, the sensitivity is a constant value, and the measured Hall voltage is a linear function of the magnetic field strength, independent of temperature, stress, etc. In practice, however, this is not entirely true. Yet, in many applications (such as for example in Hall-based linear current sensors), the absolute accuracy of the sensitivity is important. This means that the cross-sensitivities with environmental conditions like temperature, stress, etc. should be reduced, or that at least there is the possibility to compensate for them. Additionally, the dependence of the sensitivity on the Hall voltage, the latter being dependent on the applied magnetic field, implies that the sensor characteristic becomes a non-linear function of the magnetic field.
The semiconductor chip comprising the Hall element packaged in a housing is subjected to mechanical stresses which depend on environmental influences such as temperature and humidity. The varying mechanical stresses cause changes in the offset of the Hall elements, as well as changes in the sensitivity of the Hall elements due to the piezo-Hall effect. Changes in the offset are effectively suppressed using the measures described above. In order to compensate for the changes in sensitivity, it is known, for example, from DE 10154495, DE 10154498, DE 102004003853, DE 102008051949, to use a stress sensor which detects the mechanical stresses, and to use its output signal to compensate for the change in sensitivity of the Hall elements caused by the piezo-Hall effect.